1. Field of the Invention
The invention relates to a photosensitive resin and a method for patterning a substrate by use of the photosensitive resin, and more particularly to such a photosensitive resin and method suitable when far ultraviolet radiation (FUV) having a wavelength equal to or smaller than 220 nm is to be used as an exposure light.
2. Description of the Related Art
In a field of manufacturing various electronic devices such as VLSI which require minute processing in a sub-micron order, an electric device now must be more highly densified and integrated. Thus, an improved lithography technique is required for forming a minute pattern on a substrate.
One known method for forming a minute pattern is a method in which an exposure light having a shorter wavelength is used for forming a resist pattern. This method is based on Rayleigh's equation representing a resolution R or a line width of an optical system. Rayleigh's equation is EQU R=k.lambda./NA
wherein .lambda. represents a wavelength of an exposure light, NA is a numerical aperture of a lens, and k is a process factor. It can be understood from Rayleigh's equation that a wavelength .lambda. of an exposure light must be shorter in order to accomplish a higher resolution or obtain a smaller R.
For instance, an i-line which has a wavelength of 365 nm and is irradiated from a high-pressure mercury vapor lamp has been used as a light source for manufacturing a dynamic random access memory (DRAM) having an integration equal to or smaller than 64M bits. It is said that the i-line may be replaced as an exposure light source with KrF excimer laser having a wavelength of 248 nm in mass production process of 256 bits DRAM which requires a processing dimension smaller than 0.25 .mu.m. It is being studied presently to use a light source having a shorter wavelength for the purpose of manufacturing DRAM having an integration higher than 1G bit which requires a processing dimension to be smaller than 0.2 .mu.m. For that purpose, an excimer laser such as KrCl having a wavelength of 222 nm, ArF having a wavelength of 193 nm and F.sub.2 having a wavelength of 157 nm, electron beam and X-ray are considered useful according to the book "Short Wavelength Photoresist Material--Minuter Processing for ULSI" authored by Takumi Ueno, Takao Iwayanagi, Saburo Nonogaki, Hiroshi Ito and C. Grant Wilson, 1988, published through Bunshin Shuppan.
However, in view of both a target yield of mass production, in other word, a period of time for an exposure step, and an economical efficiency to be obtained when a novel light source is to be used, a photolithography technique is considered to be most useful because it can expose all objects to light at one time and much experience about the photolithography technique have been accumulated. Thus, it is necessary to develop photolithography technique which uses a light having a shorter wavelength than KrF excimer laser, that is, a light having a wavelength equal to or shorten than 220 nm. It is expected that such a technique will significantly affect a future process of manufacturing electronic devices.
An excimer laser as a light source suffers from many problems, including the short lifetime of gas from which the laser is to be irradiated and the expense of the laser irradiation apparatus. Thus, when an excimer laser is to be used as a light source in mass production of electronic devices, it is necessary to improve cost performance of the laser. Accordingly, resist material for minute processing must have a high sensitivity as well as to have a high resolution in response to more minute processing dimensions.
One known method for enhancing a sensitivity of resist is a chemically amplified resist which utilizes a photo acid generator as a sensitizer. For instance, Japanese Unexamined Patent Public Disclosure No. 2-27660 has suggested a resist comprising a combination of triphenyl-sulfonium-hexafluoro-arsenate and poly(p-tertbutoxycarbonyloxystyrene-co-.alpha.-methylstyrene). There are many reports about a chemically amplified resist as KrF excimer laser resist, for instance, in American Chemical Society Symposium Series, 1984, Vol. 242, pages 11-23, reported by Hiroshi Ito and C. Grant Willson. A chemically amplified resist is characterized by the steps of transferring proton acid, which is generated by a photo acid generator which is one of constituents of the resist, through a resist solid phase by means of post-exposure heat treatment, and amplifying chemical reaction of a resist resin due to the proton acid in a way like catalytic action up to hundreds of to thousands of times. Thus, a much higher sensitivity can be obtained relative to a prior resist which has an optical reaction efficiency, which is defined by reaction per a photon, smaller than one (1).
Presently, most newly developed resists are chemical amplified resists, and hence a chemical amplification system must be adopted in development of a high sensitive material in response to a shorter wavelength of an exposure light source.
However, if a chemically amplified resist for use with a single layer which is now widely used is to be used with an exposure light having a wavelength shorter than 220 nm for forming a minute pattern, for instance, ArF excimer laser having a wavelength of 193 nm, in general the resist intensively absorbs the exposure light. It is known that a commercially available resist would absorb most of the incident light at a surface thereof to which the light is directed, and hence the light could scarcely reach a substrate, thereby it is impossible to resolve a pattern on a substrate. Such a fact is reported, for instance, in "ArF excimer laser lithography (3)--Evaluation of Resist--", the 35th Applied Physics Institution Conference Manuscripts, 1989, 1p-K-4 by Sasago et al. Thus, in lithography having a light source comprising ArF excimer laser which is expected to be a post KrF light source, it is well known that a presently used resist cannot resolve a pattern at all.
Both a novolac resin which is a polymer in most presently used resists for use with i-line, and poly(p-vinylphenol) which is widely used as a base polymer of chemically amplified resist used for exposing a pattern with KrF excimer laser, have an aromatic ring in their molecular structure. This is because the molecular structure of a resin has to contain a lot of unsaturated bond, which is a quite strong bond, for providing a resist with resistance during a dry-etching step which is to be carried out after a pattern would have been formed in a semiconductor manufacturing process. The aromatic ring functions to achieve that purpose. Poly(p-vinylphenol) widely used in a resist for use with KrF excimer laser is transparent to KrF excimer laser. Specifically, poly(p-vinylphenol) has a transmittance of about 70% when it has a thickness of 1 .mu.m. However, poly(p-vinylphenol) intensively absorbs a light having a shorter wavelength because of an aromatic ring present in a molecular structure thereof. Thus, it is impossible to use poly(p-vinylphenol) as a lithography resist using an exposure light having a shorter wavelength, more specifically a wavelength equal to or shorter than 220 nm.
A resin which is transparent to a light having a wavelength equal to or shorter than 220 nm includes polyacryl family resin and poly(methylmethacrylate), which is hereinbelow to be referred simply as PMMA, both of which have no aromatic rings. For instance, it has been reported that a resolution of 0.13 .mu.m could be obtained by means of a stepper with ArF excimer laser using a thin PMMA film having a thickness of 0.15 .mu.m, in Journal of Vacuum Science and Technology, 1988, Vol. B6, page 1, by M. Rothschild et al.
Therefore, a polymer can be transparent to a light having a wavelength shorter than 220 nm by removing aromatic rings out of its resin structure, but, the polymer cannot obtain a sufficient resistance against a dry-etching step. Accordingly, it is impossible to use PMMA not having a sufficient dry-etching resistance as a resist to be used in electronic elements manufacturing process. There has been reported a polymer having an alicyclic alkyl group as an attempt for solving the above mentioned problem.
For instance, there has been reported a copolymer having an adamantylmethacrylate unit which is an alicyclic polymer as a polymer having a transparency to a light having a wavelength of 193 nm and further being dry-etching resistant, in Journal of Photopolymer Science and Technology, 1992, Vol. 5, No. 3, pages 439-446, by Takechi et al. and Japanese Unexamined Patent Public Disclosure No. 5-265212. For another instance, poly(norbonylmethacrylate) has been reported as a polymer having a transparency to a light having a wavelength of 193 nm and further being dry-etching resistant, in Proceedings of IEDM, 1992, CA14-18, San Francisco by M. Endo et al.
However, these methacrylic polymers both having an alicyclic alkyl group have a high hydrophobic property because an alicyclic group and further a protection group (a polarity converting group) both present in a polymer are hydrophobic. Consequently, a thin film composed of these polymers has a weak adhesion with a silicon substrate, and hence it is quite difficult to repeat the formation of uniform films. Furthermore, it is considered that such a thin film would have three problems as stated below, and hence would be difficult to use in a semiconductor manufacturing process.
1. Since a resist has a weak adhesion with a substrate, a pattern may be peeled off from a substrate or may be collapsed during or after a developing step. Thus, it is impossible to repeat the formation of the same patterns. PA0 2. Since a resin has a strong resistance against a presently used alkaline developing reagent, it is impossible to obtain a sufficient sensitivity required for resolving a pattern. Furthermore, scum tends to be generated. PA0 3. Since the above mentioned methacrylic polymer has a weak affinity with a solvent, (a) it is difficult to form a uniform layer, (b) a resin tends to precipitate out of a resist solvent, and (c) such precipitated materials cause a clean room and a wafer to be contaminated. PA0 1. Monomer having an alicyclic alkyl group such as dicyclopentenyl group and tricyclodecanyl group, and an alicyclic epoxy group such as norbornaneepoxy group. PA0 2. Monomer having a group which is cleaved by an acid to thereby induce polarity-conversion, such as tetrahydropyranyl group. PA0 3. Monomer selected from a group consisting of acrylate carboxylic acid and methacrylate carboxylic acid.
Though some reports have mentioned a polymer for use with lithography in which a light having a wavelength equal to or shorter than 220 nm is to be used, it is difficult for the above mentioned reasons to actually use a resist composed of a resin containing alicyclic alkyl groups therein in a semiconductor manufacturing process.